Resistor unit and method of making the same



June 1, 1954 c. w. KUHN ET AL 2,680,178 RESISTOR UNIT AND METHOD OF MAKING THE SAME Filed Sept. 19, 1951 2 Sheets-Sheet 1 "Fig, -3 4 4- \O 0 66 I i 36 9 6 v I 68 i I I I 8 W I 45. I I I I I 44, 5 2e, F160 1 eai I P I O 62. \f

I *9 52;: 66 Fl C1 q a INVENTORS 48v '48 48 C NCE, W. K H I 111 Q; 7 7 JEROME-Wm 4034 o R 1 BY Fla. 3 I vkb l A'r ro ELY June 1, 1954 c. w. KUHN ETAL RESISTOR UNIT AND METHOD OF MAKING THE SAME 1951 2 Sheets-Sheet 2 Filed Sept. 19,

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INVENTORS CLARENC E. W. KuHN JEROME B.WE.1V :H

ATTORNEY Patented June 1, 1954 RESISTOR UNIT AND METHOD OF MAKING THE SAME Clarence W. Kuhn and Jerome B. Welch, Wauwatosa, Wis., assignors to Cutler-Hammer, Inc.,

Milwaukee, Wis., a corporation of Delaware Application September 19, 1951, Serial No. 247,310

13 Claims. 1

This invention relates to improvements in resistor units, particularly the type used in the dynamic braking of electric motors, and to the method of making the same.

In such usage the resistor must have high capacity and requires minimum space. Dynamic braking is used extensively in diesel-electric locomotives where there are space limitations and Where forced ventilation is used. Also in such usage there is considerable vibration which ordinarily will shorten the usable life of the presently known resistors.

It is an object of this invention, therefore, to provide a resistor which: will have a high capacity; will resist destruction due to vibration; will be properly ventilated and free from damaging over-heated areas; and will require a minimum of space.

Another object of this invention is to provide a. method of constructing a grid or resistance element which: has a relatively large surface area; provides integral support for resisting warpage; has a more accurate sectional size; and has a form to give turbulence to the ventilating air without harmfully restricting the flow thereof.

In obtaining these objects use is made of a reflexed resistance ribbon element held by its loops in oppositely positioned insulators. The material employed provides good resistance per foot and resists destruction from vibration. However, ordinary ribbon resistors are likely to warp. To offset this, the transverse lengths between the loops or bends are provided with a curved cross section in shape similar to a sine wave curve. area, provides a rigid integral structural support resisting warpage, and increases the cooling effect of the air flowing through the resistor. The insulators are made in sections or blocks and are held in steel channels lined with insulating material. The blocks are held longitudinally by compression springs which keep them firmly in place and provide for expansion. The resistor has its ends Welded to suitable terminal plates. This provides a continuous electrical path without any pressure type connections.

The ribbon comprising the resistor is given the curved or sine wave cross section with a middle peak on one side of the ribbon and two side peaks (valleys) on the other side of the ribbon. Near the loops or reflexes this section is flattened abruptly into the normal section of the ribbon.

The novel features, which are considered characteristic of the invention, are set forth with particularity in the appended claims. vention itself, however, both as to its organization This shape increases the surface The in and its method of operation, together with additional objects and advantages thereof, will best be understood from the following description of a specific embodiment when read in connection with the accompanying drawings, in which:

Fig. 1 is a fragmentary view in top elevation of a resistor embodying the present invention;

Fig. 2 is a fragmentary view in rear side elevation of such resistor;

Fig. 3 is a sectional view taken on line 3-3 of Fig. 1;

Fig. 4 is an enlarged fragmentary sectional view taken on line 4-4 of Fig. 2;

Fig. 5 is an enlarged fragmentary sectional view taken on the meandering line 5-5 of Fig. 2;

Figs. 6, 7 and 8 are enlarged sectional views taken on lines 6-6, 7-1 and 8-8, respectively, of Fig. 9;

Fig. 9 is a fragmentary sectional view of a half of a length of the grid taken as indicated by the line 9-9 of Fig. 3; and

Fig. 10 is an enlarged view in side elevation of one of the shunts comprising an element of this invention.

The resistor shown in the drawings has a metal frame composed of side channels l0 and I2 and end plates I4 and I6. The end plates are welded or otherwise rigidly secured to the channel l0. Perforated flanges I8 on the end plates are removably secured to wings 20 on the channel l2 by bolts and nuts 22 or the like. The end plates may be provided with mounting feet 24 for supporting the frame in desired position. Transversely extending cover strips 26 may be welded to side channel l0 and the end plates [4 and l6.

7 The end plate [6 has a pair of spaced cups 28 opening inwardly to receive compression springs 30 for resiliently urging a pressure plate 32 toward the end plate 14.

A plurality of insulators 34 made of insulating material such as steatite are shaped so as to fit between the channels [0 and I2 and be guided by the flanges thereof. Each insulator has a plurality of spaced grooves 36 adapted to receive and hold the reflexes of the grid. These grooves are arranged in two laterally spaced rows so that a two ribbon grid may be used. In order to properly ventilate the grooves 35, ventilating slots 38 with sloping bottoms are provided to direct air passing through the grid toward and into contact with the inner surfaces of the reflexes mounted in the grooves. It is desirable to have a sheet of mica 40 or other like insulating material between the insulators 34 and the metal frame to further resist chance of short-circuiting. It is also desirable to reinforce the insulators at each end of the resistor. To do this th grooves 35 are spaced inwardly from the outer ends of the insulators and grooves 42 are used to increase surface area. The insulators 34 are held by the pressure plate 32 and springs and as expansion takes place the springs will give and prevent the insulators from cracking.

The grid "is, as shown, may be made of sections of stainless steel ribbon, for example, reflexed to provide loops or reflexes 6 at opposite ends of closely spaced transverse lengths 48. Many compositions of resistor material may be used. One suitable composition is stainless steel containing chromium, aluminum, and iron in proportions of approximately 13%, 3 /2%, and 83 /270, respectively. However, stainless steel is prone to warping and if the lengths are closely spaced to save on the size of the resistor, such warping would cause shorting between the lengths destroying the resistance value. Such warping is prevented. by giving the lengths a special cross sectional shape which has a multiple purpose. A first purpose is to provide inherent strength so that each length is in itself a strut resisting such warping or lateral deflection. A second purpose is to increase the surface area of each length to aid the cooling of each length without increasing its length. A third purpose is to provide a tortuous path for the cooling air as it passes between the spaced lengths, thus increasing its cooling effect. A fourth purpose is to drive part of the cooling air toward the inside surface of the reflexes it where they are seated in the grooves 36 to provide more cooling at these areas.

The ridges 56 and 52 extend longitudinally on opposite sides of each length to form a strutlike structure which inherently resists deflection and war-page. The height of the ridges is greater at the longitudinal center of the lengths 48 and gradually lessens until near the reflexes i6 where such height rapidly decreases so that the sectional shape becomes substantially fiat. When the ribbon is reflexed or folded over to form the loops or reflexes til, the central ridges 56 of one length are placed in juxtaposition with th valleys formed by the ridges of an adjacent length. As a result forced cooling air passing between the lengths 38 flows through a tortuous path to increase air turbulence enough to continually present to the surface of the lengths a changing mass or" air thus obtaining the maximum even heat transfer from the grid to the cooling air. The smooth. shape of the sine wave curve eliminates sharp angles and does not materially increase the frictional resistance to the flow of air. of the cooling air will also flow through the ventilating slots and thus b brought into close contact in larger volumes with the inside areas of the reflexes 55. Thus the tendency of the semi-enclosed reflexes 46 to overheat is offset and hot spots prevented. As nearly as possible uniform cooling of the grid thus obtained.

The number of rer'lexed' sections which are joined to form the complete grid M depends upon the overall resistance required of the resistor. The sections are joined by inert gas welding the ends of adjacent outer lengths using a stainless strip to make the connection solid. The outer lengths of the grid are also inert gas welded to terminal plates at of stainless steel and stainless steel strips may also be used to make the connections solid. The opposite ends of the plates 56 have lingers El? which are offset sufficiently to make such ends fit snugly ln'end grooves 39. v

The terminal plates 56 thus act as between the flanges 65 which bridge the gaps a support for the ends of the grid 44. In addi tion, these plates have depending tapped lugs 62 to which connecting cables may be connected.

In assembling the resistor, the frame is positioned so that it rests on side channel It]. Insulators 34 are installed with the pressure plate 32 held back against the compression of springs 30. The grid M with terminal plates 56 is lowered into position between the end plates 16 and i8 with the reflexes 46 and plates 56 seated in the grooves 36 of such insulator. Then the opposite insulators 34 are seated on the grid 44 with the reflexes it on the upper side and the other end of the terminal plates 56 seated in the grooves 35. During this operation the pressure plate 32 is held back. against the compression of springs 39. Then the channel I2 is placed on the upper insulators and a force of approximately four hundred pounds placed thereon to seat the refiexes it in the grooves 35 under tension. The gap between the flanges IB and the wings 20 is then measured while such force is applied. The force is thereafter removed and shims 64 slightly greater than such measured gap are inserted l8 and wings 2D and the bolts and nuts 22 tightened to hold the unit assembled. This provides proper spacing so that the grid M is securely held and yet will not be placed under a damaging pressure upon expanding when heated.

The material of the ribbon from which the grid 44 is made may, for example, have a resistance tolerance of plus or minus five per cent. However, the resistance of the completed resistor may be made more accurate, say to within one and one-half per cent. This accuracy may be obtained by varying the overall length of the ribbon and shipping some of the grooves 36. However, for ease of assembl and manufacture it is advisable to establish an overall length of the ribbon which will fill all the grooves 36- with the reflexes ii: and the terminal plates 56. In such case the final adjustment of the resistance is accomplished by the use oi a plurality of shunts between the sides of selected lengths '43. These shunts with their ends offset are provided with depending S-shaped tabs 68 which seat on the edge of the lengths 4B. The tabs 58 are welded to the lengths 48. It is advisable to have at least three or four of such shunts spaced longitudinally along the lengths All in order to evenly divide the current and eliminate as much as possible the formation of hot spots. The shunts it may be made in a variety of lengths so that various numbers of resistor lengths may be shorted out. The overall resistance is first tested and then the proper shunts selected to short out the number of lengths needed to bring the resistance down to the desired value.

Although only one embodiment of the invention is shown and described herein, it will be understood that this application is intended to cover such changes or modifications as come within the spirit of the invention or scop of the following claims.

We claim:

1. A grid for a resistor comprising a ribbon of resistance material having reflexes transverse lengths between the rail cs, each 0; id lengths having transverse cro sections of of different sizes to provide oppos vy raised longitudinally extending ridges which taper from relatively large ridges at the longitudinal center to relatively small ridges near the reflexes to thereby stiffen said lengths against deflection and cooperate with adjacent lengths to provide less tortuous paths for cooling air flowing near the reflexes.

2. A. grid for a resistor comprising a ribbon of resistance material having reflexes and transverse lengths between the reflexes, each of said lengths having cross sections consisting of straight margins at the sides joined by a curved portion having at least one peal: and one valley to provide oppositely raised longitudinally extending ridges which taper from relatively large ridges at the longitudinal center to relatively small ridges near the reflexes to thereby stiffen said lengths against deflection and cooperate with adjacent lengths to provide less tortuous paths for cooling air flowing near the reflexes.

3. A grid for a resistor comprising a ribbon of resistance material having reflexes and transverse lengths between the reflexes, each of said lengths having cross sections consisting of straight margins at the sides joined by a curved portion having one center peak on one face of lengths and two adjacent peaks on the other face of said lengths to provide oppositely raised longitudinally extending ridges which taper from relatively large ridges at the longitudinal center to relatively small ridges near the reflexes to thereby stiffen said lengths against deflection and cooperate with adjacent lengths to provide less tortuous paths for cooling air flowing near the reflexes.

4. A grid as claimed in claim 3 in which the curved portions are in registration.

5. The method of making a grid for a resistor comprising forming longitudinally spaced lengths of resistance ribbon with curved cross sections to effect opposite ridges of relatively large size at the longitudinal center of said lengths and tapering to relatively small ridges near the portion of the ribbon in between said lengths, and reflexing said last mentioned portion so that the ridges of adjacent lengths are in juxtaposition.

6. The method as claimed in claim 5 in which said forming consists of flat margins on the sides of said lengths and curved sections between said margins having peaks on opposite faces of said lengths.

7. The method as claimed in claim 6 in which said curve has a central peak on one face and two adjacent peaks on the other face.

8. A resistor comprising a grid having a series of oppositely related reflexes, oppositely positioned insulators having a series of grooves for holding said reflexes, a frame for said insulators having opposed channels in which said insulators are mounted, end plates for said frame, one of said channels being rigidly secured to said end plates, the other of said channels being adjustable with respect to said end plates, said other channel being spaced from said end plate by an amount slightly greater than the gap between said other channel and said end plates when said other channel is pressed toward said first channel to firmly hold said grid between said insulators, and means for securing said other channel as so spaced to permit said grid to expand when heated without creating any damaging pressures.

9. A resistor comprising a frame having opposed channel members, insulators carried by said channel members and having a series of grooves adapted to receive the reflexes of a ribbon-type grid, a grid having a series of reflexes mounted in said grooves to hold said grid, said grid having transverse lengths between said reflexes the sides of which are available between said channel members, and a plurality of shunts for adjusting the ultimate resistance comprising electrical connectors with oiTset ends fitting over the sides of spaced lengths; there being a number of such shunts spaced along said lengths and welded to said sides to divide up the current evenly.

10. A resistor comprising a frame having opposed channel members, insulators carried by said channel members and having a series of grooves adapted to afford support for the reflexes of a ribbon-type resistor, a resistor having a series of reflexes mounted in said grooves to hold said resistor, said resistor having transverse lengths between said reflexes, said lengths having longitudinally extending oppositely laterally projecting ridges and corresponding valleys, said ridges and valleys being of relatively large size at the longitudinal center of said lengths and tapering to relatively small ridges near said reflexes, the ridges of one length being in alinement with the valleys of an adjacent length to form less tortuous paths for cooling air flowing near said reflexes.

11. A resistor comprising a frame having op posed channel members, insulators carried by said channel members and having a series of grooves adapted to aflord support for the reflexes of a ribbon type resistor, a resistor having a series of reflexes supported by said grooves to hold said resistor, and rigid stainless steel terminal plates mounted in said grooves and having one offset end to which said resistor is welded and a finger at the other end to rigidly mount said terminals in said grooves.

12. The method of making a grid for a resistor comprising fOll'I'lll'lg longitudinally spaced lengths of resistance ribbon with curved cross sections to effect opposite ridges of relatively large size at the longitudinal center of said length and tapering to relatively small ridges near the portion of the ribbon in between said lengths, cutting the reflexeol resistor to provide the desired resistance value, and thereafter welding the cut ends of said resistor to terminal plates.

13. The method of making a grid for a resistor comprisin formin longitudinally spaced lengths of resistance ribbon with curved cross sections to effect opposite ridges of relatively large size at the longitudinal center of said length and tapering to relatively small ridges near the portion of the ribbon in between said lengths, cutting the reflexed resistor to provide the desired resistance value, thereafter welding the cut ends of said resistor to terminal plates, and skipping one or more of the supporting grooves to more evenly distribute the heating effect in the various portions of the resistor.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 534,699 Shaw Feb. 26, 1895 791,998 Ball June 13, 1905 1,065,015 Youmans June 17, 1913 1,900,318 Van Valkenburg Mar. 7, 1933 FOREIGN PATENTS Number Country Date 113,836 Australia Sept. 10, 1941 372,067 Germany Dec. 18, 1921 OTHER REFERENCES The Welding Journal, Oct. 1948, p. 827-30. 

